Study of the photochemical properties of some aromatic compounds on molecular sieves using a picosecond pulse laser system

El-Rayyes, Ali El-Said Hassan (2001) Study of the photochemical properties of some aromatic compounds on molecular sieves using a picosecond pulse laser system. PhD thesis, King Fahd University of Petroleum and Minerals.

English Abstract

Proton transfer reaction from the excited state of 1-naphthylamine (RNH₂) has been investigated in aqueous solutions of different acidity. Fluorescence from a new species was recorded. The quenching constants of: RNH₂*, RNH₃⁺* and the new species and some rate constants are evaluated by means of steady-state fluorescence and picosecond fluorescence decay measurements. The structure of the new emitting species is proposed to be an adduct formed from RNH₃⁺ and unhydrted ClO₄ anion. A theoretical model for the hydration of protons is presented. The formation of the new species is found to be in linear relationship to the presence of unhydrated acid molecules. The results of this study is used to characterize the acidity in zeolite Y and MCM-41 catalysts. Both fluorescence emission and fluorescence decay profile of RNH₂ adsorbed at the catalyst surfaces reflect high acidic environment experienced by the amine at the surfaces of the catalyst. For zeolite Y, the acidity is found to be equivalent to that in 3 M acid and increases up to 15 M by increasing the percent protonation in the catalyst. For MCM-41, the pH of the surface silanol groups is found to vary from pH 1.8 to pH 2.5. Kinetic models for the proton transfer reaction occurring at the catalyst surfaces is proposed both the fluorescence and deactivation rate constants are determined. Furthermore, the polarity of the surfaces of these catalysts is probed using 2-anilinonaphthalene (2-AN) and rhodamine b lactone (RBL). The results show that the catalyst surfaces are highly polar. The polarity could be comparable to that of an alcohol. For zeolite Y, the polarity increases upon protonation. A new fluorescence emission band from RBL adsorbed at the zeolite Y surfaces was determined. This emission band is due to a dimer formation at the catalyst surfaces. This band is not formed in solutions due to solvation effects. To our knowledge, this is the first time to characterize the surface properties using laser induced fluorescence techniques and to study the excited-state proton transfer reactions at the surfaces of zeolite Y and MCM-41 materials.